Measurement apparatus including a track for a measurement scale and apparatus for tensioning the scale

ABSTRACT

There is disclosed: a measurement scale suitable for use with scale reader for determining displacement; various tracks all for holding the scale and all having a channel into which the scale is magnetically attracted; and a scale tensioner which in use tensions the scale in situ by release of a preload, prior to the scale being secured to a substrate, the tensioner being removable thereafter.

FIELD OF THE INVENTION

[0001] The present invention relates to a track arrangement for carryinga measurement scale for use in scale, a scale therefor, and to apparatusand a method for tensioning a measurement scale.

DESCRIPTION OF THE PRIOR ART SUMMARY OF THE INVENTION

[0002] A known form of opto-electronic scale reading apparatus formeasuring relative displacement of two members comprises a length ofscale on one of the members, having scale marks defining a periodicpattern, a read head provided on the other member, means forilluminating the length of scale, periodic diffraction means forinteracting with light from the scale marks to produce interferencefringes having movement relative to the read head and detecting means inthe read head responsive to the fringes to produce a measure of thedisplacement.

[0003] An example of such apparatus is disclosed in EP-A-0 207 121 andalso U.S. Pat. No. 4,974,962. Typically, the length of scale could be ofspring steel with a surface layer of copper. U.S. Pat. No. 4,926,566discloses a method of producing a length of scale, in the form of aflexible tape produced by rolling, the pitch of the scale marks being 20μm or 40 μm for example.

[0004] Conventionally the length of scale may be carried by beingreceived by a track, for example comprising an aluminium extrusion, thelower part of the track being attached to a surface, for example byscrews or bolts, and the upper part of the track being formed to receivethe length of scale—see FIG. 1 which is a section through a length oftrack 1 which is attached to a surface 2 by screws 3 spaced apart alongits length and lengths of clamping strip 4, the upper part of the track1 receiving a length of scale 5 which has been slid into it.

[0005] The upper part of the track 1 has scale holding elements 1 a forkeeping the scale in place.

[0006] However, such an arrangement can be difficult to make and toinstall. The small ride-height tolerance of a typical read head meansthat the track 1 requires accurate manufacture. The necessary clearancefor the sliding fit of the scale 5 in the track 1 demands that the scalebe manufactured even more accurately to compensate for this clearance.

[0007] It is not desirable to clamp the scale 5 securely to the track 1because there may be relative thermally induced movement between thescale and track, particularly if dissimilar metals are used. For thisreason the prior art track shown in FIG. 1 allows sliding movement ofthe scale. The scale will produce more accurate readings if it is heldat a fixed position, both lengthwise and laterally but should not beclamped to the track, and should not be subjected to any deviations inthe forces holding it in position.

[0008] According to one aspect of the present invention there isprovided an elongate track comprising a channel, the channel beingadapted for holding a measurement scale therein, wherein the track isadapted for attracting magnetically the scale to the channel of thetrack.

[0009] The track could have magnetic material (for example a length offlexible magnetic material such as ferrite rubber) for holding the scalemagnetically. In this case, the length of magnetic material could be inthe channel of the track, the scale being above the length of magneticmaterial, for example sitting on edge portions (which are, for example,below the upper surface of the track) along respective sides of thechannel.

[0010] The track could be attached to the surface by screws passedthrough holes in it. Such holes could be at the bottom of the channel orin the track alongside such a channel.

[0011] The track could be machined or an extrusion, for example analuminium extrusion.

[0012] Alternatively the track could be a length of magnetic materialfor example ferrite rubber, having the channel formed integrallytherein.

[0013] The invention extends to a track and scale therefor.

[0014] It has been found that the scale may lift from a track or similarsupport under conditions where it is under compression. The presentinvention according to a second independent aspect relates to a scaletensioner, an embodiment of which prevents lifting of a scale.

[0015] One scale tensioning device is described in U.S. Pat. No.4,559,707 (Heidenhain). However this and other scale tensioners aredirected to correcting errors in the scale's pitch rather than toprevent lifting.

[0016] The constructional details of these prior art tensioners differfrom the proposed invention which provides a measurement scaletensioning device comprising a preloadable resilient member operable tourge the scale into tension.

[0017] The device may include a resilient member loading part operableto preload the member and operable to release the load.

[0018] The invention according to yet another aspect provides a methodof tensioning a measurement scale, comprising the following steps:

[0019] providing a measurement scale, and scale tensioner;

[0020] preloading the scale tensioner;

[0021] mounting the scale to a substrate;

[0022] securing one end of the scale to the substrate and clamping thetensioner to the substrate adjacent the other end of the scale;

[0023] releasing the preload in the tensioner to cause a tension in thescale; and

[0024] securing the said other end of the scale to the substrate whilstmaintaining the tension in the scale.

[0025] Preferably the method includes the step of providing a track tohouse the scale.

[0026] The invention extends to a method for tensioning and if necessaryrepeating the tensioning.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will now be described, by way of example,with reference to the accompanying drawings, in which:—

[0028]FIG. 1 is a section through a known arrangement for carrying alength of scale by a length of track;

[0029]FIG. 2 is a plan view of a length of track forming a firstembodiment of the invention according to the first aspect of theinvention;

[0030]FIG. 3 is an enlarged view of part of the length of track of FIG.2;

[0031]FIG. 4 is a section through the length of track and showing ascrew through a hole in it;

[0032]FIG. 5 is a section through the length of track and showing alength of scale held on it;

[0033]FIG. 6 is a section through a wider form of track;

[0034]FIG. 7 shows a use of the wider form of track;

[0035]FIG. 8 is a plan view of a length of track forming a furtherembodiment of the invention;

[0036]FIG. 9 is an enlarged view of part of the track of FIG. 8;

[0037]FIG. 10a) is a section through A-A of FIG. 9, FIG. 10b being anenlarged view of part of FIG. 10a);

[0038]FIG. 11 shows means for fixing lengths of track end to end;

[0039]FIG. 12 is a sectional view of another type of track forminganother embodiment of the invention;

[0040]FIG. 13 is a sectional view of yet another type of track formingyet another embodiment of the invention;

[0041]FIGS. 14&15 show a plan and side view respectively of a scaletensioning device according to the second aspect of the invention;

[0042]FIG. 16 is an enlarged partial view in the direction of arrow “X”in FIG. 14; and

[0043]FIG. 17 is an isometric view of the tensioning device, scale andscale track shown in FIGS. 14,15 and 16.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Referring to FIGS. 2-5, a length of aluminium extrusion providesa length of track 6 attachable to a surface by screws 7 passed throughholes 8 spaced apart (for example with a pitch of 10 cm) along thelength of track 6. The length of track 6 is formed with a channel 9running centrally along its length, the holes 8 being at the bottom ofthe channel 9. Also, along each side of the channel 9 is an edge portion10 below the upper surface of the length of track 6.

[0045] With lengths of track 6 attached to a surface by screws 7,between each pair of holes 8 there is laid a strip 11 of flexiblemagnetic material, held on to the bottom of the channel 9 by a strip 12of double-sided adhesive tape. By way of example, the strips 11 could bestrips of ferrite rubber such as that provided by Magnet ApplicationsLimited, of Northbridge Road, Berkhamsted, Hertfordshire HP4 1EH,England. Then, a length of scale 5 (for example for use inopto-electronic scale reading apparatus) is laid on to the edges 10 andheld magnetically in place on the track 6 by the strips 11 of flexiblemagnetic material.

[0046]FIG. 6 shows a section through a wider length of track 6 for usein the arrangement of FIG. 7 in which a read head carried by a sprungread head carriage 13 runs on the length of track 6. A wheel 14 is fixedvia a bracket to the read head carriage 13 and the assembly sprungtowards the scale 5. The wheel runs on the length of track 6,maintaining the correct scale to read head gap substantiallyirrespective of undulations and steps in the surface to which the lengthof track 6 is attached.

[0047] In the above embodiments, the length of track 6 is formed with achannel 9 running centrally. Referring to FIGS. 8, 9, 10 a and 10 b analternative form of length of track 6 a comprises an aluminium extrusionformed with holes 8 a spaced apart along its length for attaching thetrack 6 a to a surface by screws, the holes being offset from thelongitudinal centre line of the length of track 6 a. Also offset fromthe centre line is a channel 9 a along each side of which there is anedge 10 a. In use, the length of track 6 a is attached to a surface, astrip of flexible magnetic material is held on to the bottom of channel9 a by a strip of double-sided adhesive tape and a length of scale islaid on to the edges 10 a and held magnetically in place by the strip offlexible magnetic material. Such a structure may be used, like with theFIG. 7 embodiment, with a sprung read head carriage to which a wheel isattached, the wheel running on the upper surface of the length of track6 alongside the channel 9 a.

[0048] Such lengths of track 6 or 6 a may be placed end to end with asmall (˜0.5 mm) clearance. This is satisfactory unless the surface towhich they are attached is uneven. A solution is to make each end ofsuch a length of track as shown in FIG. 11 so two ends plug together byvirtue of dowels 15 and holes 16. In practice this could be achievedwith two keyhole slots open at the bottom, a suitable dowel-pin beingfixed in one of the holes. This enhancement would be particularly usefulwhen used in conjunction with a sprung read head carriage.

[0049] The scale may take the form shown in FIG. 12. In this embodimentan aluminium extrusion 20, having the profile shown, is used as a scalesupporting track. The extrusion 20 has a channel 22 for accommodatingheads of securement screws etc which pass through holes 24, shown inhidden detail. A further channel 26 is illustrated. Like the otherembodiments this channel 26 accommodates a scale 5 and a magnetic strip11 for holding the scale in place. The strip is held in place by meansof adhesive.

[0050] It will be noted that there is a slight concavity 28 to theunderside of the track. The purpose of the concavity is to ensure thatthe underside stays in register with the surface to which it is mounted,and does not form a convexity. A convexity would allow the track to rockon the surface, and thereby reduce control of the height of the scale.

[0051] In the above embodiments, the magnetic force holding the lengthof scale in place should not be greater than necessary since it isuseful to minimise sliding frictional force between scale and track.This is achieved by the choice of air gap between the scale and themagnetic material.

[0052]FIG. 13 shows yet another embodiment of the track. In thisembodiment a flexible magnetic strip 30 is used to hold the scale 5. Thestrip is formed from ferrite rubber of the type mentioned above. Achannel 32 is formed in the track 30 for holding the scale 5 andprovides lateral support for the scale. This track may be heldmagnetically to a ferrous substrate e.g. a cast iron machine tool bed.In use the track may be temporarily positioned for, say commissioning ofthe machine and then removed.

[0053] A variant (not shown) of the track shown in FIG. 13 has a channelslightly wider than the scale to ensure that any radii at the internalcorners of the channel do not lift the scale and distort its height.

[0054] The tracks shown in FIGS. 12 and 13 may be employed in a mannersimilar to manner of use of the other tracks described above.

[0055] It will be appreciated that the present invention may be usedwith scale and scale reading apparatus other than the opto-electronictype—for example scale and reading apparatus which operates by means ofmagnetic or capacitive detection can be used.

[0056] Other modifications and variants will be apparent to the skilledaddressee. For example the magnetic materials mentioned above might bereplaced by electromagnetic elements causing attraction of the scale tothe track

[0057] The scale may be magnetised or may have a magnetic strip affixedthereto for co-operation with a magnetic or magnetised part of thetrack. The scale may gain its magnetic attraction from the substrate towhich the track is attached, e.g. the scale may be magnetic and in useis positioned in close proximity (within the channel) to a ferroussubstrate like cast iron.

[0058] Referring to FIGS. 14,15,16 and 17 there is shown a scaletensioner which may be used e.g. to tension any of the scales describedabove. The tensioner is illustrated with a track similar to that shownin FIGS. 8,9,10 a&10 b, although a track of any type might be used andin its broadest sense the invention according to the second aspect neednot use a track. Tension in the scale prevents the possibility ofcompression and lifting of the scale.

[0059] There is shown a length of track 6 and a scale 5. The scale issecured at each end to a substrate 66 e.g. a machine tool bed. Twoclamps, one for each end of the scale, each have two parts 42 and 44.The scale is sandwiched and held between these two parts. The two clampsare secured to the substrate 66 by means of screws. The track 6illustrated is secured to the substrate also, by means of screwsextending through holes 46 in the track 6.

[0060] The tensioner comprises a main body 40 having a resilient coilspring 54 housed therein. The spring abuts a plunger 56. The spring 54is compressible within the body 40 to provide a preload and is held in acompressed state by means of an eccentric pin 52 acting to preventmovement of the plunger 56. The pin 52 is rotatable with shaft 50. Theshaft 50 has a slot 62 for manual turning of the shaft e.g. with ascrewdriver.

[0061] The body 40 is securable adjustably using clamp screw 68 to thesupport 60 which is in turn securable to the substrate via one of theholes 46 normally used to secure the track to the substrate 66, usingbolt 70. Adjustment of the position of the body 40 relative to thesupport 60 is achieved by means of a slot 48 in the body through whichthe clamp screw 68 may pass.

[0062] In use the scale and track are mounted to the substrate 66, andthe two end clamps 42 and 44 are attached to the scale. One end clamp issecured to the substrate (holding the scale) and the other is looselyattached to the substrate (again holding the scale). Support 60 issecured to the substrate via the track mounting holes 46 and thetensioner body 40 is attached to the support and slid toward the looseend clamp i.e. the clamp shown in the Figures. The body 40 in itspreloaded state is then tightened to the support.

[0063] The body is initially in the condition shown in FIG. 16 i.e. pin52 holds back the plunger 56 against the force of the spring 54. Slot 62in the shaft 50 is turned through 180° which moves the pin in aneccentric manner away from the plunger 56. This action releases thetension in the compressed spring and forces the plunger against clamppart 42 (at the abutment point 64 shown in FIG. 16).

[0064] The clamp part 42 is attached to the scale 5 so the scale will beforced into tension. The loose clamp can be tightened to the substrateand the body 40 and support 60 can be removed. The scale will remain intension. The now vacant track securement holes can be used to secure thetrack.

[0065] The preload in the spring is adjustable e.g. to give a correctlycalibrated tensioning force. A screw 66 is used to adjust the tension inthe spring to give the correct tension in the scale when the plunger isreleased. It is envisaged that the spring will produce a tension forcein the order of 10 to 40 N.

[0066] The process of scale tensioning may need to be repeated,particularly when a long scale is being tensioned. Long scales willstretch further than short ones when subjected to the same tension, sothe stroke of the pin 52 may be insufficient to induce the requiredtension in the scale.

[0067] In such circumstances the spring 54 can be compressed again byturning shaft 50 and the body 40 can be repositioned to abut once moreclamp part 42 at abutment 64. The scale clamp is loosened. The preloadin the spring 54 is released by turning the shaft 50 and the scale isthereby tensioned once more.

[0068] The clamp is then tightened to the substrate.

[0069] The process may be repeated until no more extension is observedwhen the spring is released.

[0070] The period of the marking etc used on a scale under tension mayneed to be shorter than those on a non tensioned scale because the scalemay stretch. A scale with a finer pitch may therefore be provided foruse with this tensioner.

[0071] Other tensioner modifications and variants will be apparent tothe skilled addressee e.g. the spring 54 may be replaced with a gasoperated piston, in which case its preload may come from compression ofentrapped gas behind the piston, or from a pressurised supply of gas. Ineither case the piston can be held back under preload by a restraintmechanism e.g. a pin similar to the pin 52.

1. A measurement scale tensioning device suitable for tensioning a scalereadable by a readhead while the scale is mounted to a substrate, thedevice comprising; a preloadable member operable to urge the scale intotension while the scale is mounted to the substrate.
 2. The measurementscale tensioning device of claim 1, further comprising a securement thatsecures each end of the scale to the substrate, and a further securementthat secures the device to the substrate while the device is inoperation.
 3. The measurement scale tensioning device of claim 1,wherein the preloadable member includes a resilient part, and thetensioner includes a loading part that preloades the member and thatreleases a preload.
 4. The measurement scale tensioning device of claim3, wherein the loading part includes an eccentric, rotatable to effectthe preloading and release of the preload.
 5. A method for tensioning ameasurement scale comprising the steps of: providing a measurement scaleand a scale tensioner; mounting the scale to a substrate; securing oneend of the scale to the substrate; preloading the scale tensioner;securing the tensioner to the substrate adjacent another end of thescale; releasing the preload in the tensioner to cause a tension in thescale; securing the other end of the scale to the substrate whilemaintaining the tension in the scale; and releasing the securement ofthe tensioner.
 6. The method of claim 5, further comprising the stepsof; preloading the scale tensioner again; re-securing the tensioner tothe substrate adjacent the other end of the scale; releasing the preloadin the tensioner; releasing the securement of the scale at the saidother end, thereby allowing further tension in the scale; re-securingthe said other end of the scale to the substrate, whilst maintaining thefurther tension in the scale; releasing the securement of the tensioner;and repeating the aforementioned steps until the scale extendssubstantially no more on application of the further tension.
 7. Themethod of claim 5, further comprising the step of removing the tensionerfrom the substrate following securement of the scale at the other end ofthe scale.
 8. The method of claim 6, further comprising the step ofremoving the tensioner from the substrate following re-securement of thescale at the other end of the scale.
 9. The method of claim 5, whereinthe step of securing the scale to the substrate includes the provisionof a track to hold the scale to the substrate.